Furnace and furnace wall



July 27, 1965 J. R. BEATTIE 3,197,185

FURNACE AND FURNACE WALL Filed April 30, 1963 3 Sheets-Sheet 2 Inventor B MWd L/ July 27, 1965 J. R. BEATTIE FURNACE AND FURNACE WALL 3 Sheets-Sheet 3 Filed April 30, 1963 m \9 Y V 6 \\\\A, 5 C 4 Q. j J m w w m w w w w m m 0 mmem 7/1 Mmm JT/MW/ 4 B K United States Patent 3,197,185 FURNACE AND FURNACE WALL John Reginald Beattie, Maghull, near Liverpool, England, assignor to Pilkington Brothers Limited, Liverpool, England, a corporation of Great Britain Filed Apr. 30, 1963, Ser. No. 276,833 Claims priority, application Great Britain, May 3, 1962, 17,082/62 6 Claims. (Cl. 26340) This invention relates to furnaces for use in the heat treatment of materials or mixtures of materials.

While a furnace according to the invention may be used for heating metals or mixtures of metals, for example in powder form, at temperatures within a wide range, it has particular application in a temperature range from about 800 C. to 1000 C. Another application of the invention is in the heat treatment of glass when it is frequently desired to heat glass articles near to the softening point of the glass, so that the desired operating temperature of the furnace used is in the neighbourhood of 800 C. A furnace according to the present invention is particularly suited to the heat treatment of glass at such a temperature, and the heating of flat glass in sheet form to this temperature is one of the steps in the conventional toughening process.

The furnaces which have generally been used hitherto for this purpose have been refractory furnaces which have a high heat capacity and which consequently require a considerable expenditure of heat to raise them to the desired operating temperature in the neighbourhood of 800 C.

In the heating of flat glass in sheet form for the toughening operation, it is desired to obtain a predetermined temperature or temperature pattern in the glass, for example a uniform temperature throughout the glass, before subjecting the glass to a chilling operation. However, if it is found that the flat glass has local temperatures departing from the predetermined pattern and it is desired to correct for these differences by heating a portion of the glass by further heating in the appropriate portion of the furnace the response of a refractory furnace to the application of further heat is very slow because of the high thermal capacity of the refractory furnace.

In other fields, particularly the heat treatment of metals, it has been proposed to use furnaces having a cavity wall structure. However, the materials used for the cavity wall structure in such furnaces, for example molybdenum or chromium-plated stainless steel, have all been liable to oxidation at the operating temperature of the furnace, and the oxidised materials have such low coefiicients of reflection (for example 20%) that the advantage of a low thermal capacity which is conferred by the cavity wall structure, is nullified by the much greater loss of heat to the surroundings on account of the impaired thermal insulation in the walls of the furnace.

Accordingly it is a main object of the present invention to provide an improved furnace construction which has the advantage of low thermal capacity and high speed of response to local applications of heat in the furnace during operation of the furnace, without seriously impairing the thermal insulation provided by the walls of the furnace.

According to the present invention there is provided a heat treatment furnace comprising a cavity wall structure formed by a plurality of spaced partitions characterised by one or more of the spaced partitions being coated on at least one surface with a coating comprising a refractory material to establish a layer on that surface having a surface reflectivity greater than its surface absorptivity at the operating temperatures.

3,197,185 Patented July 27, 1965 Amongst the refractory materials we particularly mention calcium fluoride, magnesium oxide, zinc oxide, calcium oxide, alumina, lead oxide, and zirconia including 5% of calcium oxide, as materials for use in the present invention.

According to the present invention there is more particularly provided a furnace for use in the heat treatment of flat glass comprising a cavity wall structure formed of a plurality of spaced partitions, characterised by one or more of the spaced partitions being coated on at least one surface with a refractory material to establish a layer on that surface of the partition having a heat reflectivity greater than its heat absorptivity, whereby the thermal insulation of the wall structure at the operating temperature of the furnace is improved and there is provided a consequent greater speed of response to local changes in the heat applied in the enclosure within the cavity wall structure.

The refractory material is preferably a refractory oxide material.

Advantageously the refractory material has a heat reflectivity greater than its heat absorptivity at wavelengths below 6 microns and at the operating temperature of the furnace.

The presence of the refractory material or refractory oxide material according to the present invention prevents the oxidation of the metal or alloy of which the partitions are composed, the reflectivity of the oxides of the metals of the partitions being frequently as low as 20%.

Heat is supplied to the furnace by heating devices nor mally associated with the side Walls of the furnace. The heating devices may be electric heaters or gas burners and, by the use of the present invention, the radiated heat emitted by the heating devices in the direction of the side walls of the furnace is returned from the cavity wall structure which comprises the furnace walls to the centre of the furnace where the glass being heated is situated.

According to one aspect of the present invention, therefore, there is provided a heat treatment furnace comprising a cavity wall structure including a plurality of spaced partitions and heating devices adapted to direct heat to the interior and associated with the innermost partitions, characterised by one or more of the spaced partitions being coated on at least one surface with a coating comprising a refractory material to establish a layer on that surface having a surface reflectivity greater than its surface absorptivity at the operating temperature of the furnace, whereby there is redirected to the interior of the furnace heat radiated from the heating devices to the innermost partitions.

More particularly according to this aspect of the invention there is provided a furnace for use in the heat treatment of flat glass comprising a cavity wall structure including a plurality of spaced partitions, and heating devices adapted to direct heat to the glass and associated with the innermost partitions, characterised by the innermost partitions having on at least one surface a coating of a heat reflecting refractory material having the characteristic that its surface reflectivity is greater than its surface absorptivity at the operating temperatures of the furnace whereby there is redirected to the glass heat radiated from the heating devices to the innermost partitions, whereby the thermal insulation of the Wall structure is improved.

The refractory material is material.

Conveniently the spaced partitions are assembled as individual cavity wall structures each assembly forming a side wall of the furnace.

In order to be suitable for coating the spaced partitions in a furnace according to the present invention, it is not preferably a refractory oxide necessary for the refractory material or refractory oxide material to have a heat reflectivity greater than its heat absorptivity over the whole range of infra-red wavelengths which are emitted by the heating devices in the furnace. A very high proportion of the radiations emitted by the heating devices will be at wavelengths below microns and as much as 86% of the radiation emitted at 888 C. by a perfect radiator is at wavelengths below 6 microns; Accordingly the essential feature of the refractory materials or refractory oxide materials is that they shall have heat reflectivity greater than their heat absorptivity at a substantial proportion of Wavelengths between 1 and 6 microns. Preferred examples of refractory oxide materials are ziconia including 5% of calcium oxide, magnesium oxide, and alumina.

From the preceding discussion, it will be appreciated that a coating of a refractory material, for example a refractory oxide material, will be effective when the material is applied to the inner surface of one of the partitions comprising a cavity wall structure in a furnace according to the present invention. However, the characteristics of the refractory materials that the heat reflectivity is greater than the heat absorptivity also means that their heat emissivity will be less than 50% so that the provision of a layer of the refractory material on the outer wall of one or more of the partitions will reduce the heat lost by radiation from that partition in an outward direction.

In general, the heating devices in the furnace are heating elements which are mounted on the innermost partition and it is convenient in such a case that there shall be no application of a layer of a refractory material to the inner face of the innermost partition, but layers of the refractory material are preferably applied to the outer face of the innermost partition and the inner face of the partition next to the innermost partition.

Conveniently there is also applied a layer of refractory material to the outer face of the said next partition so that the present invention comprehends the presence of a layer of refractory material on both surfaces of the individual partitions.

The invention also comprehends a cavity wall structure formed of a plurality of heat reflecting partitions mounted in spaced relation, the heat reflecting surfaces of at least one of the spaced partitions being comprised by a layer of a refractory material, for example a refractory oxide material, having a heat reflectivity greater than its heat absorptivity, for use in manufacturing a furnace according to the invention.

Preferably the refractory material has a heat reflectivity greater than its heat absorptivity at wave lengths below 6 microns and at the operating temperature of the furnace.

The invention will be more clearly understood from the following detailed description of a preferred embodiment thereof, taken in conjunction with the accompanying drawings in which:

FIGURE 1 shows a longitudinal section of a furnace according to the invention for use in heating sheets of glass to be toughened,

FIGURE 2 is a cross-sectional view of the furnace of FIGURE 1 taken along the line IIII, and

FIGURE 3 shows the percentage reflection of two refractory oxides in comparison with the percentage reflection of an oxidised metallic material over a range of wavelengths.

In the drawings, like reference numerals designate the same or similar parts.

Referring first to FIGURES'I and 2 of the accompanying drawings, the furnace has side walls indicated generally by the reference numeral 1, each of the side walls being formed individually of a series of heat reflecting partitions or radiation shield-s 2, 3, 4, 5 and 6, the heat reflecting partitions 2 to 6 are held in spaced relation by refractory spacers 7, which are carried by respective rigid frames 8. The rigid frames 55 hold the partitions 2 to 6 forming each side wall as a unitary structure.

The three innermost partitions 2, 3 and 4 are made of the refractory metallic material obtainable under the name Nimonic 75, which is a binary alloy consisting of 80% nickel and 20% chromium and stiffened by the precipitate of titanium carbide caused by the presence of about 0.3% of titanium and 0.1% of carbon. The partition 5 is stainless steel and the partition 6 is an outer sheet of aluminium. Although the individual partitions 2 to 6 are shown in the drawings as being plain sheets, they may, if desired, be formed in a corrugated manner in order to increase their rigidity. The spacing between the individual partitions is of the order of 1" to '2".

The innermost partition 2 of Nimonic carries on its outer surface a layer 9 of zirconia plus 5% calcium oxide and the next partitions 3 and 4 have on both their inner and outer surfaces similar layers 10 and 11, 12 and 13 of zirconia plus 5% calcium oxide. 7

The layers 9 to 13 applied to the surfaces of the partitions 2-, 3 and 4 are preferably about .005" in thickness and are particulate coverings in nature rather than films of zirconia plus 5% calcium oxide. The layers 9 to 13 may conveniently be deposited on the surfaces of the partitions 2, 3 and 4 as an atomised spray, so that the particles of zirconia plus 5% calcium oxide sinter together on the surfaces of the partitions 2, 3 and 4 to form the respective layers.

in FIGURE 3 there is shown as curve A the percentage reflection of zirconia plus 5% calcium oxide against the wavelength in microns of radiations falling on it. It will be seen that for the portion of the spectrum in which the majority of radiations are emitted, that is to say below 5 microns, the percentage reflection of zirconia plus 5% calcium oxide is in excess of50% over a substantial part of the range.

As curve B in FIGURE 3, there is shown the percentage reflection of an oxidised sheet of Nimonic 75 and it will be seen that the reflectivity of the zirconia plus 5% calcium oxide layer causes a very substantial increase in the reflectivity of the surfaces of the Ni-monis 75 partitions 2, 3 and 4 to which the zirconia plus 5% calcium oxide layers 9 to 13 are applied.

Also in FIGURE 3 there is shown as curve C the percentage reflection of alumina.

Referring again to FIGURE 2 of the drawings, there are shown heating element-s 14- of nickel-chromium strip which are supported by refractory bodies 15 mounted on the innermost partitions 2 of the longitudinal walls of the furnace. The voltage supply for the heating elements 14 is provided through a transmission conduit (not shown) which will pass through the several partition-s forming the walls ll from a suitable terminal box,

In the embodiment of the invention shown in FIG- URES 1 and 2, the base of the furnace consists of a stepped foundation 16 of refractory material, the stepped edges of the foundation I6 mating with corresponding stepped edges on the bases of the individual side walls 1. The furnace has an apertured roof structure 17, also of refractory material, and the roof structure is lodged on the side walls 1, the junction between the roof structure and the side walls also being of a stepped nature. The innermost partitions 2 extend inwardly to the'vicinity of the aperture 17 in the roof, and there are provided slidable closing members 18 by which the furnace may be completely closed when a glass sheet has been lowered into it for heating near to the softening temperature of the glass.

In operation, the heating elements '14 radiate heat inwardly to a glass sheet 19 within the furnace and also outwardly to the innermost partitions 2 of the unitary structures forming the walls ll. Consequently the innermost partitions 2 are heated and themselves radiate heat both inwardly and outwardly. However, in consequence of the layer 9 of zirconia plus 5% calcium oxide, the

quantity of energy radiated by the partition 2 outwardly to the next innermost partition 3 is lower than would be the case in the absence of the layer 9.

The presence of the layer on the next partition 3 increases the reflectivity of the surface of the partition 3 so that a greater quantity of the radiation emitted by the surface of partition 2 in an outward direction is reflected back to the partition 2. However, of all the radiation which falls on partition 3 and heats partition 3, a smaller proportion is radiated outwardly from the partition 3 due to the presence on its outer surface of the layer 11. The layers 12 and 13 on partition 4 act in a similar manner to the layers 10 and 11 on partition 2,

As a result of the presence of the layers 9 to 13 on the partitions Z, 3 and 4, the loss of heat outwardly through the furnace Walls is reduced.

Consequently it is possible to maintain the furnace according to the present invention at the desired temperature with the expenditure of less heat energy than would be the case for a similar furnace, but without the layers 9 to 13.

A furnace according to the present invention also has a high speed of response to changes in heat input to the furnace so that it is possible to compensate fairly rapidly for variations in temperature in the individual zones within the furnace by adjusting the heat input to the furnace, for example, from one of the heating elements 14.

Although the present invention has been described as applied to a furnace into which a sheet of glass is lowered, it will be appreciated that it may equally be applied to a furnace through which a glass sheet is carried on a vehicle. Furthermore, if desired, the foundation and the roof of the furnace may both be cavity wall structures similar to the side walls 1.

In addition a furnace according to the present invention may be used for heating glass articles in order to decorate them by fusing coatings of colouring materials on their surfaces.

Although the coating of zirconia plus 5% calcium oxide is conveniently deposited as an atomised spray, it is more convenient with some of the other refractory materials, to apply them as a slurry or paste which is baked onto the base metal of the partition to form the particulate covering. Also it is within the ambit of the invention to apply the refractory material or refractory oxide material as a mixture with another material, but the mixture must be such that the particulate covering obtained has the properties hereinbefore set out.

I claim:

1. A heat treatment furnace having a low thermal capacity and high speed of response to local applications of heat and exhibiting good thermal insulation at temperatures in the range of 800 to 1000 C., the furnace comprising side furnace Wall defining a furnace interior, heating devices capable of heating said furnace to said temperature range and of maintaining said furnace at 9 said temperature range and so positioned within the furnace interior as to direct radiant heat to the interior of said furnace, each of said furnace walls consisting of a plurality of spaced metal partitions forming a cavity wall structure, at least one of the spaced partitions having a coating of refractory material forming a reflecting surface, said refractory material being such, that in the temsurface reflectivity greater than its surface absorptivity at perature range of 800 C. to 1000" C., said material has a surface reflectivity greater than its surface absorptivity at a substantial proportion of wavelengths between 1 and 6 microns for inhibiting the transmission of heat through the cavity wall structure from the interior of the furnace;

2. A heat treatment furnace having a low thermal capacity and high speed of response to local applications of heat and exhibiting good thermal insulation at temperatures in the range of 800 C. to 1000" (3., the furnace comprising side furnace walls defining a furnace interior, heating devices capable of heating said furnace to said temperature range and of maintaining said furnace at said temperature range and so positioned within the furnace interior as to direct radiant heat to the interior of said furnace, each of said furnace walls consisting of a plurality of spaced metal partitions forming a cavity wall structure, at least one of the spaced partitions having a coating of refractory material selected from the group consisting of calcium fluoride, magnesium oxide, zinc oxide, calcium oxide, alumina, lead oxide and zirconia including 5% of calcium oxide, said coating forming a heat reflecting surface inhibiting the transmission of heat through the cavity wall structure from the interior of the furnace.

3. A furnace according to claim 1, in which the refractory material is a refractory oxide material.

4. A furnace according to claim 1, wherein the cavity wall structure is on each side of the furnace, and each cavity wall structure has an innermost partition with its outer surface coated with said refractory material and a next partition with its inner surface coated with said refractory material.

5. A furnace according to claim 4, in which said next partition also has a coating of said refractory material on its outer surface.

6. A furnace according to claim 1 for the heat treatment of flat glass.

References Cited by the Examiner UNITED STATES PATENTS 1,871,937 8/32 Wilson 263 X 2,577,184 12/51 Dietrichet a1 219-3413 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,197,185 July 27, 1965 John Reginald Beattie s in the above numbered pat- It is hereby certified that error appear 1d read as ent requiring correction and that the said Letters Patent shou corrected below.

Column 3, line 7, for "888 C." read 800 C. column 6, line 8, strike out "surface reflectivity greater than its surface absorptivity at".

Signed and sealed this 8th day of February 1966.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

1. A HEAT TREATMENT FURNACE HAVING A LOW THERMAL CAPACITY AND HIGH SPEED OF RESPONSE TO LOCAL APPLICATIONS OF HEAT AND EXHIBITING GOOD THERMAL INSULATION AT TEMPERATURES IN THE RANGE OF 800* TO 1000*C., THE FURNACE COMPRISING SIDE FURNACE WALLS DEFINING A FURNACE INTERIOR, HEATING DEVICES CAPABLE OF HEATING SAID FURNACE TO SAID TEMPERATURE RANGE AND OF MAINTAINING SAID FURNACE AT SAID TEMPERATURE RANGE AND SO POSITIONED WITHIN THE FURNACE INTERIOR AS TO DIRECT RADIANT HEAT TO THE INTERIOR OF SAID FURNACE, EACH OF SAID FURNACE WALLS CONSISTING OF A PLURALITY OF SPACED METAL PARTITIONS FORMING A CAVITY WALL STRUCTURE, AT LEAST ONE OF THE SPACED PARTITIONS HAVING A COATING OF REFRACTORY MATERIAL FORMING A REFLECTING SURFACE, SAID REFRACTORY MATERIAL BEING SUCH, THAT IN THE TEMSURFACE REFLECTIVITY GREATER THAN ITS SURFACEE ABSORPTIVITY AT PERTURE RANGE OF 800*C. TO 1000*C., SAID MATERIALA HAS A SURFACE REFLECTIVITY GREATER THAN ITS SURFACE ABSORPTIVITY AT A SUBSTANTIALLY PROPORTION OF WAVELENGTHS BETWEEN 1 AND 6 MICRONS FOR INHIBITING THE TRANSMISSION OF HEAT THROUGH THE CAVITY WALL STRUCTURE FROM THE INTERIOR OF THE FURNACE. 